The invention relates to a magnetic disk apparatus and a read waveform equalizing method in which after a read waveform from a disk medium was equalized by using a transversal filter, bit data is detected in accordance with a Viterbi algorithm and, more particularly, to a magnetic disk apparatus and a read waveform equalizing method in which a transversal filter is controlled to an optimum state in accordance with a state of a disk medium.
In magnetic disk apparatuses, there are requests to realize a high density and a high speed. Such requests will be more and more increased in future. To satisfy the request to realize a high density, an improvement of a magnetic recording medium, an improvement of a magnetic head, a decrease in circuit noises, and the like are performed. In a magnetic disk apparatus of a type in which the magnetic head is floating from a disk, a floating amount of the magnetic head from the disk is also an important factor for realization of a high density. That is, when the floating amount of the head is decreased, an output and a resolution rise in accordance with the decreased floating amount. On the contrary, however, when a dust or the like enters between the head and the disk, a probability of the occurrence of a damage of the head is large and there is inevitably a limitation in the minimum value of the floating amount.
Therefore, in future, there is a limitation in the improvement of the output and resolution in the magnetic head--medium system. It is demanded to optimize a performance of an equalizer for equalizing a read waveform so that it can cope with a low S/N ratio and a low resolution.
Hitherto, in the magnetic disk apparatus, a waveform read out from a disk is analog differentiated and its zero-cross point is detected as a peak position. However, in a system of a low resolution, namely, a system in which isolated waves in a reproduction signal are neighboring, a peak shift such that the peak of a reproduction waveform is deviated from an actual peak of the isolated waveform appears in the reproduction waveform due to a waveform interference of the adjacent isolated waves. Thus, the reproduction waveform is erroneously seen as if information was written to the adjacent bits.
To prevent such a problem, as a method of improving such a drawback when data is read, a cosine equalizing circuit (pulse slimming circuit) is used to emphasize a high frequency component of a signal and improve a resolution. As a cosine equalizing circuit, there are a method of equalizing a waveform on a time domain and a method of equalizing a waveform on a frequency domain. The former method is generally used.
A cosine equalizing circuit for approximately performing a cosine equalization on a time base is constructed by a delay line of a delay time .tau., a voltage divider, and a differential amplifier. When a read voltage is given to an input terminal, the voltage which was delayed by only the time t is applied to a (+) input of the amplifier. A voltage of a peak value (K) and a voltage of the peak value (K) having a delay of 2.tau. which was transmitted through the delay line and reflected by the differential amplifier and inputted are supplied to a (-) input, so that an addition synthesized output is obtained. Therefore, an output voltage of the cosine equalizing circuit is a voltage obtained by slimming the read waveform. By using such a cosine equalizing circuit, frequency characteristics in which a high frequency component is emphasized are obtained. However, when the high frequency component of the signal is emphasized, a wide band component of noises are also simultaneously emphasized, so that a jitter increases.
As a method of improving when data is written, there is also performed a write compensation such that a peak shift amount is predicted from the beginning and, at a time point when data is written to a medium, the data is alternately delayed or advanced, thereby narrowing a data interval than the actual waveform. However, a process to narrow the data interval is substantially the same as a process such that a recording density on the medium is raised and the data is written, resulting in decrease in S/N ratio and resolution.
Even in a system of a reduced resolution, therefore, a circuit or a method which can accurately read out the signal is necessary. A system in which a waveform interference exerts an influence on the adjacent bits can be regarded as a kind of convolution code. A Viterbi decoding method known as a maximum likelihood detecting method for the convolution code is used in the field of the magnetic recording.
Although the Viterbi decoding method is fundamentally advantageous in the system of a low resolution, in case of actually applying the Viterbi decoding method to the magnetic disk apparatus, there is a problem about how to fetch the value of the isolated waveform at a sampling point. Generally, recording densities at the inner and outer rims of the magnetic disk apparatus differ and shapes of isolated waveforms also differ. Therefore, in order to accurately perform the Viterbi detection, it is necessary to accurately fetch at a high speed the data of the isolated waveform on each track from the innermost rim to the outermost rim. The above problem is solved by providing a transversal filter as an equalizer for eliminating an intercede interference at the front stage of the Viterbi decoder.
Hitherto, as a magnetic disk apparatus using the Viterbi decoding method, for example, there is an apparatus disclosed in JP-A-5-314654. According to such an apparatus, an automatic equalizer is provided at the front stage of a Viterbi decoder, thereby performing an optimum equalization of the read waveform. The automatic equalizer has a transversal filter and optimizes tap coefficients by a learning function. When the tap coefficients are set, an output error of the automatic equalizer is obtained by using an equalization target pattern stored in a memory and the tap coefficients are controlled so as to minimize such an error.
As initial values of the tap coefficients which are set to the automatic equalizer, preset information is recorded every track or sector of a disk at the time of a shipping or a turn-on of a power source, and the preset information is read out and, in a manner similar to the case of the reproduction, tap coefficients which minimize an error with the equalization target pattern are obtained and stored in the memory. Just before the reproduction, the memory is accessed and the initial values of the tap coefficients are set to the automatic equalizer.
Such a method whereby the automatic equalizer and Viterbi decoder are combined is nothing but the technique of the communication field is applied as it is. In the communication field, for example, there is a time-dependent fluctuation of transmission path characteristics such as a frequency selective fading in a radio transmission. To eliminate an intercode interference which follows such a fluctuation, there is used an automatic equalizer to automatically realize equalization characteristics by using a training signal which is sent prior to the data transmission.
In case of the magnetic disk apparatus, however, transmission path characteristics which are obtained by a path comprising a disk medium, a head, a head IC, and a read amplifier are fairly stable as compared with those of a communication system. Particularly, there is no need to consider a time-dependent fluctuation of the transmission path characteristics in a short time such as a frequency selective fading. There is a temperature change of the apparatus as a factor of the time-dependent fluctuation of the transmission path characteristics. Although such a temperature change causes a physical fluctuation of a head positioning control or the like, the transmission path characteristics are not so largely influenced.
Therefore, in the automatic equalization such that an error between an equalization output during the reproduction and an equalization target pattern is obtained and tap coefficients of a transversal filter are feedback controlled so as to minimize such an error, since there is no special fluctuation factor in the transmission path comprising the disk medium, head, head IC, and read amplifier, so long as the initial set tap coefficients are proper, no error eventually occurs. A necessity of the automatic equalization is low. There is, consequently, a problem such that in case of the magnetic disk apparatus, the use of the automatic equalizer causes a complication of a circuit construction and a complication of the control due to the automatic equalization.
The automatic equalizer of the conventional apparatus always controls the tap coefficients so as to minimize the equalization error by using the equalization target pattern of the memory even during the reproduction. However, there is a problem such that when the tap coefficients are controlled during the reproduction, the control of the tap coefficients traces a large noise mixture when a shock or vibration is applied, abnormal tap coefficients are set, and the equalization cannot be performed.